1. Field of the Invention
The invention relates to a metal strip stretching mill which has a stretching station and, arranged oppositely adjacent thereto, a group of pulling rolls and a group of braking rolls with at least two wrap-around rolls each, the pulling roll and the braking roll in the immediate vicinity of the stretching station having a controlled circumferential speed, and the other rolls having a torque-controlled drive with a power input which is stepped down in accordance with the distance of the roll from the stretching station.
2. Description of the Prior Art
Metal strip stretching mills of this type are used advantageously for the stretching of aluminum strip.
In this type of metal strip stretching mill, the rolls in the immediate vicinity of the stretching station have a controlled circumferential speed in order to thereby determine the overall speed of the mill and its stretch ratio. The other rolls are equipped either with separate, controllable drives or with differential drives, their torque input being determined as a function of their relationship to the rolls in the immediate vicinity of the stretching station. Electric motors for this kind of separate drive or differential drive of the individual rolls have normally a torque control range over their armature current of approximately 1 to 10. The individual rolls of each group contribute a stepped torque to the strip tension. At each successive roll, this tension is decreased or increased, as the case may be, by a fixed ratio of approximately 2.
One of the two groups of rolls must provide a step-down from the stretch tension to the drag tension of the supply shaft, the other from the stretch tension to the winding tension of the winding shaft. As a result, such an installation is controllable for reproducible process conditions only within a power range of 1 to 10. This signifies that, for such a mill, the lowest power input acceptable must not lie below 10 percent of the maximum power input, or design power input.
Accordingly, when there is a need for processing thin, narrow and soft strip stock for which less than 10 percent of the design power output is required, it cannot be accomplished with such an installation. On the other hand, it is also not possible to simply disconnect individual drive motors, because of the sizeable power comsumption of their drive train elements, especially their branching drives. The power losses under these circumstances would lead to completely undefinable situations.